In general, a VCR is provided with a drum motor to rotate the rotary head drum, a capstan motor to transfer a video tape and a servo circuit for controlling the rotational speed of these motors.
In a typical prior art servo system such as the one illustrated in FIG. 1, a frequency generator 4 is adapted to periodically generate a frequency signal FG corresponding to the rotational speed of the motor 2 each time the motor is rotated by a predetermined angle (for example, 30.degree.). The frequency signal from the frequency generator 4 is supplied to a circuit 12 for detecting the speed error of the motor 2 by comparing the frequency signal with a predetermined reference frequency or speed signal to produce a speed error signal. The speed error signal is provided to a PID (Proportional Integration Differential) controller 16 and then to an amplifier 18 for driving the motor. The output of the amplifier 18 is used as a speed control signal and the speed of the motor is thereby controlled.
However, since most frequency generators have certain degrees of manufacturing tolerance, a deviation between the reference signal and the periodic frequency signals generated by the frequency generator may cause the production of a speed error signal in spite of a constant rotational speed of the motor. Consequently, it is rather difficult to precisely control the rotational speed of a motor.
In recent years, a repetitive learning servo system for controlling the rotational speed of the motor has been proposed. One of such systems is described in an article by Makoto Gotou, et. al., entitled Development of Multirate Sampling Repetitive Learning Servo System and its Application to a Compact Camcorder, IEEE/RSJ International Workshop on Intelligent Robots and Systems IROS '91, Nov. 3-5, 1991, Osaka, Japan, IEEE Cat. No. 91TH0375-6.
This repetitive learning servo system compensates repetitively the period deviation of the periodic frequency signals to make it zero after the lapse of a certain period. The compensated signal may be used as a speed control signal comparable to the speed control signal which is employed in the typical system shown in FIG. 1.
However, control through the use of the repetitive learning servo system may entail a worse result than that of the traditional servo system in certain situations. For instance, in the event that the rotational speed of the motor is changed by certain external factors so that it is constantly maintained for a long time at the state of the changed speed, the repetitive learning servo system will fail to correctly control the rotational speed of the motor because the average period deviations would become zero at the changed speed.